1. Field of the Invention
The present invention is directed to processes for control of NO.sub.X emissions. More particularly, the present invention is directed to improved reburning processes for reducing NO.sub.X emissions from combustion systems such as power plant boilers, process furnaces, and incinerators. By advanced air and fuel staging techniques, the present invention provides apparatus and methods for optimum conditions for the injection of selective reducing agents in the burnout zone and enhances the efficiency of reburning for NO.sub.X control.
2. Technology Review
Combustion of fossil fuels, especially coals and heavy oils, produces significant amount of NO.sub.X which ultimately participate in the formation of smog and acid rain. Combustion modification concepts including staged combustion and reburning have been effective in achieving up to 60 percent NO.sub.X reductions. Downstream injection of selective reducing agents, particularly ammonia ("NH.sub.3 ") and urea ("CO(NH.sub.2).sub.2 "), can produce significant additional reductions. However, the temperature window over which these reagents are effective is relatively narrow.
Reburning is a NO.sub.X control process which uses fuel to reduce nitric oxide ("NO"). In the reburning process, a fraction of the fuel, between 10 and 20 percent of the total heat input, is injected above the main heat release zone to produce an oxygen deficient reburning zone. Hydrocarbon radicals from combustion of reburning fuel react with nitric oxide to form molecular nitrogen, thus reducing NO. This process occurs best in the absence of oxygen. Subsequently, burnout air is injected downstream to combust the remaining fuel fragments and convert the exiting HCN and NH.sub.3 species to either NO or N.sub.2.
Previous studies have shown that 60 percent reduction in NO.sub.X emissions can be achieved with natural gas reburning and that most of the reduction occurs in the reburning zone. NO.sub.X, reduction in the burnout zone, via the HCN and NH.sub.3 species from the reburning zone, is minimal because of the high burnout temperature (2200.degree. F.-2400.degree. F.) and the presence of excessive amount of carbon monoxide ("CO", above 2 percent at 0.9 stoichiometry).
In practice, NO.sub.X reduction by reburning is considerably less than the limits imposed by thermodynamics on NO formation under fuel-rich conditions. Examination of equilibrium levels of total fixed nitrogen species (HCN+NH.sub.3 +NO) as a function of stoichiometry (SR) and temperature suggests that, except for high temperatures (greater than 3100.degree. F.) and extremely fuel-rich conditions (SR&lt;0.5), the equilibrium levels are less than 10 ppm. Equilibrium considerations would dictate operation of a combustor at moderate temperatures under fuelrich conditions, such as the reburning zone, or low temperatures near stoichiometric conditions.
From the foregoing, it is apparent that what is currently needed in the art are methods for optimizing reburning processes used to control NO.sub.X emissions from combustion systems.
Such methods are disclosed and claimed herein.